The present invention generally relates to separation membranes and, more particularly, to ozone resistant separation membranes for Nitrogen Generating Systems (NGS).
Separation membranes have been used in air separation modules (ASM) of NGS, such as On Board Inert Gas Generating Systems (OBIGGS). OBIGGS have been used in aircraft to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen enriched air (NEA). The OBIGGS passes air (e.g. bleed air) through the ASM, generating the NEA and a stream of oxygen enriched air (OEA). The resulting NEA can be used to inert fuel tanks while the OEA can be vented overboard or recaptured.
Separation membranes are based on permeable membrane (PM) technology. Permeation of a gas, such as oxygen, through a membrane is generally considered to be the product of the diffusivity of that gas through the membrane and the solubility of that gas in the membrane material. Oxygen is separated from the bleed air because the membrane is more permeable to oxygen than to nitrogen. As the bleed air travels through the ASM, the NEA flow is generated by the loss of oxygen via permeation through the separation membrane.
At cruising altitude, the ozone concentration of the air can reach 1 ppm or higher. Under such conditions, the separation membranes made of polymeric materials tend to break down due to oxidation. Membrane deterioration negatively affects the gas separating properties of the membrane. Various methods for removing ozone from the air stream have been described in the prior art.
In aircraft, ozone removal has been accomplished by passing the air through a bed of carbon granules. Unfortunately, carbon and similar ozone removal materials are not fully acceptable for some applications since these materials are consumed in the filtration process and it is often difficult to ascertain when these materials have reached the end of their useful life. Additionally, the carbon beds are heavy and they create a high pressure drop.
Ozone decomposing catalysts have been used for many applications, such as air purification systems and water disinfection applications. When compared with the ozone removal materials, catalysts have much longer operating lives and generate less waste.
In U.S. Pat. No. 4,348,360, a catalytic converter for ozone removal in aircraft is disclosed. The described catalytic converter comprises an aluminum honeycomb core, a tubular metal shell surrounding the core and affixed thereto, metal rib support members fastened to the shell and to the honeycomb core, and a catalytic coating deposited upon the cell walls of the honeycomb core.
In lieu of the honeycomb core described in the '360 patent, other catalytic converters for ozone removal have included substrates made from fibers, pellets, or particles which are metallic or a metal coated ceramic material.
Catalytic converters for ozone removal have been used in aircraft inerting systems. In U.S. Pat. No. 7,152,635, an inerting system for an aircraft is disclosed. In the described system, an ozone converter is positioned upstream of the ASM to improve life and reliability of the ASM. If the ozone converter system fails or provides incomplete ozone conversion, ozone can pass to the ASM and damage the separation membrane.
In U.S. Pat. No. 5,422,331, a layered catalyst composition is disclosed. The described composition contains an undercoat layer that provides adherence to substrates and an overlayer on which is dispersed one or more catalytic metal components. The composition is described as especially well adapted for use in aircraft. In air handling systems, the substrate on which the layered catalyst composition is coated need not be a honeycomb-type carrier or other substrate specifically configured to support a catalyst, but the layered catalyst composition could be applied to any portion of the air handling system in which the air sustains turbulent flow. Thus, the layered catalyst composition may be applied to the blades of an air handling fan or compressor, to a grill, louvers or other air-directing structure, or on other structures over which the air of the air handling system is forced in turbulent flow. For some applications, suitable substrates may not be available. Additionally, the reactor temperature or residence time may not be sufficient to provide complete ozone conversion. In some situations, the catalytic material applied to substrates specifically configured to support the catalyst (e.g., honeycomb carriers) or to system substrates (e.g., compressor blades) is not sufficient to provide complete ozone conversion. Furthermore, ozone conversion in the catalytic system can never reach 100%. Under these situations, ozone can damage the separation membranes of the NGS, which in turn can lead to dangerous conditions within the fuel tank.
As can be seen, there is a need for an ozone resistant membrane or an improved ozone removal system. There is also a need to improve ozone removal from the air stream and reduce separation membrane damage caused by ozone.